Low cetane, i.e., high octane, fuels, such as natural gas, have several advantages over other hydrocarbon fuels that are combusted in internal combustion engines. For example, natural gas is less expensive relative to other hydrocarbon fuels. Moreover, natural gas burns cleaner during operation of the internal combustion engine relative to other hydrocarbon fuels. By burning cleaner, a reduced amount of combustion byproducts such as carbon monoxide, oxides of nitrogen, and hydrocarbons are released into the environment during engine operation. In addition, because lubricants of the internal combustion engine become contaminated with combustion byproducts over time, the production of a reduced amount of combustion byproducts results in less contamination, thereby increasing the useful life of the lubricants.
One type of internal combustion engine is an auto-ignite engine, such as a typical diesel engine. Diesel engines combust fuel by compressing a mixture of air and fuel to a point where the fuel is ignited by heat, which results from such compression. When natural gas is used as a fuel in an auto-ignite engine, the natural gas does not readily ignite as it is compressed. In order to overcome this problem, an ignition source is provided to ignite the natural gas, such as a spark plug. In other types of engines, e.g., dual fuel engines, the ignition source is provided by injecting a small amount of pilot fuel, such as diesel fuel, into a mixture of air and natural gas (or other non-auto-igniting fuel). As the mixture of air, natural gas, and pilot fuel is compressed, the pilot fuel ignites, which in turn provides an auto-type ignition of the natural gas.
A disadvantage associated with using pilot fuel as an ignition source is the resulting generation of an increased amount of oxides of nitrogen (NOx). In particular, the ratio of air to the combination of natural gas and pilot fuel in the combustion chamber varies with the proximity to the injected streams of pilot fuel. Rich mixtures are created near the location of injection of pilot fuel, while lean mixtures are created further away from the location of the injection. Combustion of the rich mixtures tends to produce more NOx than does the combustion of the lean mixtures.
One way to reduce the amount of NOx produced during the combustion process is to create a lean homogeneous mixture of air, natural gas, and pilot fuel throughout the combustion chamber prior to ignition. Because the homogeneous mixture is lean throughout the entire combustion chamber, only lean mixtures are combusted. Combustion of only lean mixtures produces a lesser quantity of NOx than does combustion of a combination of rich mixtures and lean mixtures. Once ignition is desired, a spark plug may be used to ignite the lean homogeneous mixture.
In commonly-owned U.S. Pat. No. 6,666,185 to Willi et al. (“Willi”), Willi discloses a method and apparatus for controlling the injection of pilot fuel to control ignition of a homogenous distribution of fuel in the engine. The method and apparatus of Willi comprises adjusting the injection timing and quantity of pilot fuel to control ignition as a function of engine load. In Willi, the fuel is auto-ignited.
In the present disclosure, a method and apparatus for controlling ignition of a homogenous distribution of pilot fuel and primary fuel using a spark plug is provided.